Electron beam lithography equipment typically includes a beam column, acting as the generator for producing the exposure beam, and a moveable support table for moving the work piece to be exposed by the beam past the beam. As the surface of the work piece passes under the beam, the beam is modulated to form desired marks on the surface of the disk. Such electron beam lithography equipment, for example, has been suggested for use in forming servo marks on the master for the production of magnetic disks used in hard disk drives. However, application of beam lithography to rotating disk shaped work pieces presents certain problems, as will be discussed below.
A magnetic disk drive, such as a hard disk drive, stores data on one or more disks coated with a magnetic medium. For read/write purposes, the surface of the magnetic medium carries a number of generally parallel data tracks, which on a disk type medium, are arranged concentrically with one another about the center of the disk. An actuator arm positions a transducer or “head” over a desired track, and the head writes data to the track or reads data from the track. As the disk rotates, the actuator arm moves the head in a radial direction across the data tracks under control of a closed-loop servo system, based on position information or “servo data,” which is stored within dedicated servo fields of the magnetic medium of the disk. The servo fields can be interleaved with data sectors on the disk surface or can be located on a separate disk surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a readback signal that identifies the location of the head relative to the center line of the desired track. Based on this location, the servo system moves the actuator arm to adjust the head's position so that it moves toward a position over the desired track and/or a desired location within the track of current interest. Systems for forming the servo tracks on a master for the production of magnetic disks have used both stepped translation mechanisms with laser beams and continuous translation mechanisms with electron beams.
Generally, beam lithography equipment controls the velocity of movement of the work piece under the beam to obtain a desired exposure dose. However, for rotating disk type applications, such as servo mark formation on master disks, it is advantageous to rotate the disk work piece under the beam and to move the disk work piece radially under the beam, during exposure. Assuming the beam remains stationary, the combination of the rotation and radial translation of the disk work piece causes the beam to expose a spiral pattern. In another application (US Publication No. 2004/0001415; entitled Manufacture of Concentric Patterns From Spiral Source), the inventors have developed a technique using cyclical deflection of the beam in synchronization with disk rotation, to convert the spiral pattern to a series of concentric rings. The beam is modulated on and off during such movement of the disk to form a series of desired servo marks along the spiral or along the concentric rings.
During such beam lithography processing of disk work pieces, it is advantageous to rotate the work piece at a constant rotational speed (constant angular velocity—CAV). This provides coherence between successive passes (or tracks). Good coherence, or low track-to-track phase error, is a requirement for good servo track writing so as to support good drive servo performance, for example in disk drive applications.
However, by using CAV during exposure of a rotating disk work piece, the linear velocity of the disk surface at the point under the beam increases proportionally with the radius from the center of disk rotation. Since the linear speed varies with radius, the resulting dosage also varies, since by the beam energy and the linear speed determine the dosage at any given point. As speed increases, the dosage per unit area decreases. Such variation in dosage results in unwanted variation in the feature geometry.
It may be helpful to consider FIG. 11 as an example. The drawing shows a recording disk 101, e.g. a resist coated silicon wafer, having a center opening 103. As the disk 101 rotates and translates relative to the recording beam, the beam is modulated to form servo marks in the photoresist surface, in patterns 105.
The enlarged view of region 107 shows a representative area of the disk work piece, and thereby makes visible a number of the actual marks 109, in a region relatively near the center opening 103. In the region 107, the radial distance from the center of the disk 101 is relatively small. In a similar fashion, the enlarged view of region 111 shows a representative area of the disk, and thereby makes visible a number of the actual marks 113, in a region relatively near the outer edge of the disk 101. In the region 111, the radial distance from the center of the disk 101 is relatively large.
Because the angular rotation rate is constant (e.g. constant RPM), the linear velocity of the disk surface increases at points further from the center of disk rotation. The greater the radius from the center, the greater the circumference at that distance, and the higher will be the linear speed relative to the exposure by the beam. As a result, a greater arc of the disk surface passes under the beam during a unit of exposure time. Any unit of exposed area therefore receives less exposure dose because it is moving faster (because at a greater radius from the center).
FIG. 12 shows the timing of the actual beam-on pulse 115, in relation to the exposed areas 109 and 113 in the regions 107 and 111. The on-time of the beam used to expose each mark is the same, as represented by the high state of the pulse 115. However, as noted above, the linear velocity increases with radius, since the disk 101 rotates at a constant angular velocity during processing. As a result, as the linear speed increases (higher radius), the length of the marks increases (compare 109 to 113). The two different lengths of marks 109, 113 illustrate the linear change in feature length, from outer diameter (OD) to inner diameter (ID), when marks are formed using a constant substrate rotation rate and a constant mark exposure time. Under such a processing scenario, actual exposure dose is effectively stretched over a longer mark area as radius increase. Hence, the exposure dose in a given unit of surface area decreases as radius increases (or the dose in a given unit of surface area increases with decreasing radius).
Like many other beam lithography applications, servo pattern formation on a master for the production of magnetic disks requires uniform exposure doses. That is to say, the exposure dose in any exposed unit of area of the master disk should be held substantially constant across the exposure surface.
It might be possible to vary the electron beam current to compensate for variations in linear velocity. Unfortunately, variation of the beam current adversely affects the spot size and again produces unwanted changes in the recorded feature geometry.
A need therefore exists for a technique to adapt the beam lithography approach, for example using the rotation and translation of the disk work piece, e.g. a resist coated silicon wafer forming a master disk, so that the lithography produces a substantially uniform exposure dosage for marks regardless of the location (e.g. radially) on the disk surface, without causing other unwanted changes in the recorded feature geometry.